Printing plate mounting and proofing machines have been disclosed, for example, in U.S. Pat. Nos. 2,493,628, E. L. Harley; 2,561,115, E. L. Harley and 5,058,287, R. Harley. The machines of these patents, however, were not specifically designed for mounting and proofing plates for printing corrugated boxed.
One type of conventional printing plate mounting and proofing machine especially adapted for proofing printing plates for printing of blanks for corrugated boxes, has been provided with an upper or layout cylinder upon which a paper layout sheet for drawing a layout pattern may be mounted by conventional means, and a lower or plate cylinder upon which printing plates are mounted for proofing.
In one technique for using a machine of this type for mounting and proofing printing plates for a corrugated box printing press, a sheet of paper was initially mounted on the upper cylinder of the machine, and a full size layout of the box blank was drawn on the paper. The machine was provided with means for enabling the drawing of vertical and horizontal lines on the paper corresponding to determined positions of a corrugated box blank to be printed. Thus, in a conventional process, the user is enabled to draw lines on the paper corresponding to the outlines of the blank, the score lines of the blank, and the locations for printing plates. The printing plate locations generally correspond to the centers of the printing plates, since printing plates are usually provided with marks indicating the central axes thereof. In the drafting of the layout of the box on the layout sheet on the layout cylinder, the dimensions of the plan correspond to the full size of the box. This full size layout was used to mount the prepared printing plates in the desired pattern on the plate cylinder.
In such a machine, it was generally desirable to employ a plate cylinder of the same diameter as the printing cylinder that was to be used in a printing press. If the plate cylinder was not of the same diameter as the printing cylinder, a problem arose in the location of the printing on the box blank, resulting from "stretching". Thus, plates are manufactured to have the proper dimensions for a printing cylinder of a determined diameter. If the curvature of the printing cylinder is greater than design curvature, i.e. if the diameter of the printing cylinder is smaller than the design diameter of the printing plate, then, due to its finite thickness, it will stretch when it is mounted on the printing cylinder. For example, if a plate is designed for a 66 inch cylinder, and it is used on a 50 inch cylinder, the print that results from use of the plate will be elongated. Similarly, if the curvature of the printing cylinder is greater than the design diameter, the pattern on the plate will be compressed when it is mounted on the printing cylinder. The thickness of the printing plate and backing of the plate are also factors in the amount of stretch that will occur.
A similar problem occurs when more than one plate is to be mounted on the printing cylinder, for example at spaced apart locations such that the impression of each plate appears on a different side of a finished box. In this case, if the curvature of the printing cylinder is different than the curvature of the plate cylinder on the proofing machine, the impressions from the different plates that are printed on the box blank will be spaced apart by distances such that the impressions do not appear at the desired locations on the side of the box.
It is for this reason that it has been desirable to provide a plate cylinder that has the same diameter as the printing press cylinder. This solution results in the problem, however, that it becomes necessary to frequently change the plate cylinder on the proofing machine, in order to avoid the necessity for compensating for stretch. The plate cylinders, however, may be quite large and heavy, so that changing them on the proofing machine is difficult and time consuming.
In the above solution to the problem of stretch, the layout on the layout cylinder was designed to be of the same size as the pattern on the box, with the same spacings, etc. In another solution to the problem, it has been suggested that the layout pattern be made of a size that compensates for stretch, so that a single size of plate cylinder may be employed for all sizes of printing press cylinders. For this purpose, the layout pattern was prepared on a flat table, using a "stretch" ruler on which the calibrations were adjusted to compensate for stretch. For example, on a ruler that was designed for layout on a proofing machine having a 66 inch circumference, to proof plates for use on a printing press cylinder having a circumference of 50 inches, a calibrated ruler is employed to dimension the layout, wherein the actual dimensions are different than the dimensions indicated on the scale of the ruler. In this solution, a separate calibration scale or ruler must be employed for each size combination of cylinders, and the accuracy of stretch compensation is limited by the difficulty of accurately employing the markings on the scale.
In a further solution, it has been suggested that the proofing machine be provided with an additional plate cylinder on the side thereof opposite the first plate cylinder. In this solution, the two plate cylinders have different diameters, to correspond to the diameters of the two printing press cylinders that are used most often. While this solution reduces the need to change plate cylinders as often, it greatly increases the cost and size of the proofing and mounting machine, and does not provide any advantage when printing press cylinders have other dimensions.
A further problem exists in the use of the plate mounting and proofing machine, in that the sizes of the various printing cylinders are not standardized, and considerable variation exists in the sizes of cylinders that are nominally of the same size. Thus, a cylinder that is nominally called a 66 inch cylinder may in fact have a circumference of, for example, 65 to 67 inches. Such variation of course reduces the accuracy of any prior solution to the problem of compensating for stretch.